General Carter Ham, head of the US military's Africa Command, said there were signs that Boko Haram in Nigeria, al-Shabaab in Somalia and al-Qaeda in the Islamic Maghreb are sharing money and explosive materials and training fighters together.

"Each of these three organisations is by itself a dangerous and worrisome threat," Ham told an African Centre for Strategic Studies seminar in Washington. "What really concerns me is the indications that [they] are seeking to co-ordinate and synchronise their efforts."

Al-Shabaab is active in Somalia and has also been blamed for attacks in Kenya.

Last month the US classified three of the alleged leaders of Boko Haram, an Islamist sect based in remote northeast Nigeria, as "foreign terrorists", but it declined to blacklist the entire organisation to avoid boosting the group's profile internationally.

Al-Qaeda in the Islamic Maghreb, an affiliate of al-Qaeda based in North Africa, is mainly a criminal organisation operating in the Sahel region.
It kidnaps Westerners for ransom and fuels Africa's drug trade, according to intelligence officials.